witching hour

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An hour when supernatural events are thought to occur, especially midnight.
• noun The hour before the closing of the stock market on the third Friday of the month, when two or more classes of options or futures expire, known for its high trading volume and volatile prices.

from the GNU version of the Collaborative International Dictionary of English.

• noun colloq. The middle of the night, especially midnight.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The hour after midnight, when witches and other supernatural beings were thought to be active, and to which bad luck was ascribed.